Austin Franklin wrote:
> > As many people probably realize, in a typical rear curtain/focal plane
> > film cameras (as most 35mm SLRs are), any shutter speed beyond the
> > maximum flash synch shutter speed exposes the film via a moving slit
> > opening between the shutter curtains.
>
> I know what you say CAN be certainly true for the highest speeds of some
> cameras, but I did not know it was specifically related to the synch
> speed...I believe it's more related to shutter design than specifically tied
> to sync speed. Would you mind citing a source for that information?
>
> That is certainly not the case with vertical shutters, which all but one of
> my 35mm cameras have (Contaxes and Nikons), the exception being my Leica M.
>
It _is_ related to the synch speed, because electronic flash is so fast that
it needs the entire image area exposed when the flash goes off. If the camera
speed is set above the synch speed, then the "moving slit" effect means that
only that portion of the film exposed by the "slit" at the moment of flash
will get the benefits of the flash. The "flash-lighted" area will then be
correctly exposed, while the non-lit area will be heavily under-exposed.
Note this only applies to electronic flash guns, which give very short
duration flashes - typically 1/30,000 sec. The old fashioned flash bulbs
"burn" much more slowly and give light for long enough for the "slit" to do
it's full run across the film.
The maximum synch speed is set at a value at which the leading shutter blind
has completed it's run across the film before the following blind starts
moving. The first blind completes it's travel and exposes all the film area,
the flash goes off and lights it, then the second blind runs across to cover
the film area again. In most focal plane shutters this is about 1/60 - 1/125
sec. Cameras with an inter-lens shutter do not suffer with this problem, as
the shutter opens and shuts with a single action, avoiding the "moving slit"
effect.
Brian Rumary, England
